KETV-TV reports at least one of the children went to Children’s Hospital and Medical Center in Omaha.

Parents received a letter Tuesday alerting them to the medical issue. On Wednesday, doctors at Children’s let one of the recovering students go home.

“It’s scary,” said Jamie Wheeler, a Marine Simons Elementary parent. “My kids are going to school. I want them safe at school. That’s the last place you think that they are going to get anything like that.”

The first case appeared Oct. 1. Three other cases occurred on Oct. 7. So far, no one knows where the salmonella came from.

Most parents said they knew about the sick children before the district sent its letter Tuesday.

“My son is in fifth grade and he told me that one of his friends was sick with it,” Wheeler said.

Superintendent Jay Lutt said he called the district’s food supplier. He said the supplier hasn’t found any evidence of salmonella or any cases related to its food products. Because the source remains a mystery, parents want faculty to be extra careful.

At home, parents are focusing on personal hygiene.

“My son, he’s really good at washing his hands anyway,” Wheeler said. “I just hope that he does that (at) school, washing his hands whenever he goes to the restroom.”